Coding tube for pulse code modulation signals



2 Sheets-Sheet 1 ATTORNEY July 25, 1950' R. L. CARBREY CODING TUBE FORPULSE CODE MODULATION SIGNALS Filed-Sept. 30, 1948 July 25, 1950 R. L.CARBREY CODING TUBE FOR PULSE CODE MODULATION SIGNALS Filed Sept. 30,1948 2 Sheets-Sheet 2 Patented July 25, 1956 UNITED STATES orncs CODINGTUBE FOR PULSE CODE MODULATION SIGNALS Application September 30, 1948,Serial No. 52,088

9 Claims.

The present invention relates to the automatic generation of pulses incode combinations indicative of instantaneous signal amplitude, underthe direct control of signals to be coded. More specifically, theinvention relates to automatic generation of code pulses by an electronbeam coding tube in which the beam when deflected by different amountsby difierent signal amplitudes is caused to impinge upon certain ones ofa set of electrodes in difierent combinations corresponding to thedifierent digits of the code and to set up pulses on these electrodes.

Objects of the invention are, increased speed of coding, simplificationof apparatus requirements, and improved operation.

Electron beam coding tubes of the kind to which the invention isapplicable have made use of a quantizing grid and a feedback controlfrom this grid to the beam deflecting means, for limiting the beamposition to the grid aperture to which the beam was moved by the signaland for preventin the beam from assuming a position overlapping two gridapertures or only partially illuminating the intended grid aperture.Each grid aperture is lined up with a horizontal row of apertures in acode plate. Thus, registry of the beam with an aperture in thequantizing grid also causes proper registry of the beam with a given rowof code apertures. The digit electrodes are located back of the codeplate so that i the beam in passing through apertures in the platestrikes the corresponding digit electrodes.

In the use of such tubes a time considerably longer than the duration ofa code pulse is allowed for the stabilizing of the beam position,

under control of the quantizin grid and its feedback action, in properregistry with a given row of code apertures. During this stabilizingperiod the beam is illuminating the digit electrodes and so producingpulses on them. In order to prevent these pulses from reaching theoutput circuit until the beam is stabilized and the pulses are wanted,gating amplifiers individual to the digit electrodes have been used andthese ampliflers have been biased beyond cut-off until such time as thepulses were to be sent.

In accordance with this invention, the gating function is performedwithin the tube by blanking the electron beam used for the codin untilsuch time as the pulses are to be impressed on the output circuit of thecoding tube. In order to make use of the feedback principle to bring andhold the beam in proper registry with a given row of code apertures, anauxiliary, or pilot beam is provided together with a relatively narrowquantizing grid arranged in the path of the pilot beam but out of thepath of the main or coding beam. The same beam deflecting means servesin common for both beams. This allows the feedback control, through theagency of the pilot beam, to establish the necessary conditions on thedeflecting means such as will cause proper registry of the main beamwith a given row of apertures when the main beam is unblanked. The mainbeam may be turned on at the instant when the code pulses are to begenerated and not until then. It is left on for the duration of a codepulse and is then turned off until the next code combination is to begenerated.

' Thus, the main beam is used both to generate the pulses in proper codecombinations and to time the occurrence and duration of the pulses. Oneadvantage following from this action is that the gating amplifiersheretofore used may be dispensed with and if amplification as such asneeded it can be supplied in a common amplifler, for example in theoutput of the distributor as will appear from the fuller description tofollow.

The nature and objects of the invention will appear more clearly fromthe following detailed description of certain illustrative embodimentsof the invention as shown in the accompanying drawings.

In the drawings, Fig. .l is a perspective view of the electrodestructure of an electron beam coding tube of one form according to theinvention;

Fig. 1A shows a detail of the electron gun used in the constructionshown in Fig. 1;

Fig. 2 is a schematic plan view of the electrode structure of a modifiedelectron beam tube according to the invention;

Fig. 3 is a schematic view, in plan, of a tube of the Fig. 1 typeshowing the manner of connecting the tube into a pulse code transmittingsystem; and

Fig. 4 shows a timing diagram to be referred to in the description ofFig. 3.

Fig. 1 shows the electrode structure of a cathode beam tube, lookingaway from the small end of the tube at which the beam-forming means isprovided toward the large end of the tube where the target electrodesare located. Provision is made for producing a pilot beam 30 ofrelatively narrow ribbon form and a main or coding beam 3| also ofribbon form and much wider than beam 30. These two beams impinge uponthe code plate 36 and illuminate a horizontal line across the codeplate, as shown.

The code plate 36 is divided functionally into two distinct parts. Thefirst part comprises the quantizing grid 31 which is in the path of thepilot beam 30 but is to the left and out of the path of the main beam3|. The other portion comprises the coding section of the plate 36, thisportion being in the path of the main beam 3| but out of the path ofbeam 30. It will be understood that the two beams 30 and 3| aremaintained spaced apart from each other and quite distinct and separatein their effect. Positively biased concentrating electrodes l, 2 and- 3are provided to shape the electron beams and to assist in maintainingthe spacing. thereof.

All of the apertures in the code plate 316 are arranged in horizontalrows. In a seven-digit system, assumed in this figure, there will be-128 horizontal rows. The quantizing grid 31 has 128 apertures arrangedin a vertical column.

Just back of this column of apertures is the r the plate 3t differ innumber in eachhorizontal row'from none to a maximum of seven inaccordance with a binary system to form a permutation code and arepositioned in seven vertical columns. A few of these apertures are shownat 38. The digit electrodes 4| to 41', inclusive, are located just backof the cod'epl'ate 38- in position to be struck by portions of the beam31- which pass through aperturesin the correspond-- ing column.

The beams 36 and 3! are shown in the figure in their middle positioncorresponding to zero applied signal They can be deflected upward ordownward to any chosen horizontal row of apertures by the applicationofproper voltage to the deflecting plates 32", 34. These voltages may beapplied over the leads 49 and 5! respectively: 7

Any suitable type of electron gunstructure may be provided for producingthe beams 30 and 31 in ribbon form. The structure shown in Fig. 1 is amodification of that shown in United States patent to A. M. Skellett2,293,567 granted August I8, 1942. The structurein Fig. I differs fromthat in the Skellett patent in that electrostatic focusing is used inFig. 1 in place of magnetic focusing shown in the patent. The

electron gun comprises an elongated linear cathode 22, which may be ofthe indirectly heated equipotential type as shown or may be of otherforms, for example, filamentary, a cy- 4 duce desired focusing. Portionsof the shield are broken away in Fig. 1 to show the two grids 23 and 33as distinct and separated from each other. If desired, suitable shieldmeans may be included in the space between the two grids within theshield member 24.

In the operation of the electrode. structure of Fig; l a signal sampleis applied across the leads it, 5! for the deflector plates 32, 34. (Thecircuit arrangements for carrying out the functions in this briefdescription will be given in Fig. 3.) During the time the signal sampleis applied to the deflector plates themain beam 3! is blanked by theapplication of a proper voltage to the grid 23', but the pilot beam 38is not blanked at this time. The pilot beam is therefore deflected.under control of the signal voltage to the vicinity of a particularaperture in the quantizing grid 3'! depending upon the amplitude of thesignal. The beam may fall into exact registry with one of theseapertures or may be only partially: in register with an aperture andpart-way between two apertures. The portion of the beam that passesthrough an aperture strikes the target electrode Ml from. which avoltage is fed back through an amplifier to the deflector plates 32, 3such as to exert feedback control on the: pilot beam- 30 This feedbackcontrol is such as to shift the beam 33 slightly when necessary tochangefrom the condition of partial registry of the beam with a givenapertureinto full registry of the beam with that aperture. (The mannerin which the feedback control effects this result is disclosed morefully in an article by R. W. Sears entitled Electron Beam Deflection'I-u-be for a' Pulse Code Modulation published in the Bell SystemTechnical Journal for January 1948, pages 44 to 57'.) If the beam ispartly cut off on its upper side so that only, say; its lower half istransmitted, then ifthe feedback acts so that stronger current toelectrode ill urges the beam downward (and weaker current urges itupward) an unstable condition exists and the beam will either movedownward until further movement would begin to cut off' its lower edgeor it will: move upward i'indrical grid 23 for the main beam and asimilar 7 grid 33 for the pilot beam, each coaxial with the cathode 22,and a shield electrode including a U-shaped portion; 24' coaxial withthe cathode 2-2; This shield portion has attached to it a pair ofcoplanar plates 25' extending normal to the longitudinal axis of thetube or of the beam and spaced from each other to define an elongatedlinear aperture 0r slit 26, parallel to and to a stable position in thenext higher aperture.

The positioning of the pilot beam in the man.- ner described so as to bein full regiflry with a particular aperture in the quantijzing grid 3'!results in the establishment of such voltages on the deflector plates 34that when the main beam is turned on it will immediately strike thecorresponding horizontal row of holes in code plate 36 in full registrytherewith. It thus becomes practicable to turn on the main beam for justthe short interval corresponding to the duration of a code pulse andthen turn the beam off again, since-the q'uantizing gridand feedbackaction through the agency of the pilot beam has prepared in advance theproper voltage condition on deflector plates 32', 34 for causing correctregistry of the main beamwith the corresponding row of code determiningapertures. Portions of the main beam pass through these apertures andstrike the corresponding digit electrodes 41- to 4'? producing voltageson those particular; and only those, digit electrodes.

' Thefact that the two electron beams 30 and 3| are of widely differentdimensions does not prevent the type of control just described fromtaking place. The important thing is to have the accelerating potentialsfor the two beams maintained equal and this result is achieved bygenerating the beams from a common cathode the digit electrodes 4| to 41may be made use of in any desired manner. For example, they may besimultaneously transmitted over individual transmission channels or theymay be applied to a distributor operating to arrange them in sequentialorder suitable for transmission over a single channel.

Referring to Fig. 3, the signal which may be speech or other suitablesignal is shown as comingin on lead 93 to the signal sampling circuit92a} The latter may be an electronic switch of the type which permitsconduction in either direction when closed. Such switches are sometimesreferred to as two-way clamps. The switch would be biased open toprevent conduction except for the time during which a signal sample isto be taken and applied to the storage condenser 95. The timing controlfor the sampling circuit 92 and for other functions to be carried outwould in practice be accomplished by means of electronic timing circuitsunder the prime control of a crystal oscillator of highly constantfrequency, with the various timing periods determined bycapacitance-resistance timing circuits operating in conjunction withvacuum tubes in a manner generally well known in the art. For simplicityof illustration in Fig. 3 the timing control is shown as obtained fromrotating brushes IOI, I02, I03 cooperating with segmented rings. Anegative voltage from battery I05 is shown as applied to the commonshaft I04 for the three brushes. The sampling circuit 92 is biasedopenduring the portion of the cycle in which the brushis in contact with theconducting portion 98 of the segmented ring but this bias is removedduring the time the brush IN is in contact with the insulating segmentI06. It is during this latter interval that the sampling circuit 92 isallowed to take a sample of the impressed signal in lead 93 and store itupon the condenser 95.

During all or the greater part of this sampling period both the mainbeam 3| and the pilot beam 30 are suppressed by the application of highnegative voltage to the grids 23 and 33 by brushes I03 and I02 incontact with respective conducting segments I00 and I01. The pilot beamis shut off during this period to allow the voltage conditions onstorage condenser 95 to become stabilized. The high negative bias isremoved from grid 33 near the end of the sampling period, allowing the.

pilot beam 30 to become projected.

The signal voltage stored on the condenser 95 isamplified in thepush-pull amplifier 9| and is applied over the leads 49, 5| to the upperand lower deflecting plates 32, 34. The pilot beam 30 is thus deflectedto an extent determined by the signal amplitude. The quantizing grid 31,electrode 40, feedback connection II to the input of amplifier 9|,amplifier 9| and deflector plates 32, 34 cooperate in the manner abovedescribed to position the pilot beam 30 in full registry with theaperture corresponding to the nearest quantized value of the signal. Aconsiderable time is allowed during which this feedback quantizingoperation is being carried out in order to permit the pilot beam tobecome fully stabilized in its finalposition. During this period thebrush I03 is traversing the segment I00 and maintaining high negativebias on grid 23 thus suppressing the main beam. After a time suficientfor stabilizedconditions to be reached, brush I03 arrives as quiteshort." After the projection of the main beam both beams are againsuppressed and switch IOI permits sampling circuit 92 to take a newsample of the applied signal.

These timing relations are shown for further illustration in Fig. 4. Itis assumed for illustration inFig. 4 that-the coder is being used in a96-channel telephone system with speech samples taken at the'rate of8,000 per second. The coding period for any one speech sample istherefore 1.3 microseconds. Beginning at time 0 in Fig. 4

the sampling periodis shown as lasting for about half a microsecond. Thepilot beam is shown as coming on near the end of the sampling period andlasting throughout the remainder of the first 1.3-microsecond interval.Experience shows that it requires somewhat less than 0.5 microsecond forthe action of the quantizing grid and its feedback control to becomefully stabilized. The main beam is represented as not being turned ontherefore until near the end of the 1.3-microsecond interval. The codertherefore produces a main beam ulse near the end of every coding periodand pulses of the duration of the main beam pulse, shown as .186microsecond, will appear simultaneously on those of the digit electrodes4| through 41 for which the beam was in registry with apertures in thecoding plate 36. Typical code groups of pulses obtained by distributionin time of the simultaneously appearing pulses are shown in Fig. 4 forchannels 96 and I.

Fig. 3 shows a type of distributor suitable for use in distributing thecode pulses sequentially in time so as to make them suitable fortransmission over a single channel. This is done by the use of delaycircuits 88, 89, etc. individual to the seven-digit targets 4| to 41.These delay lines are associated with the respective digit targetsthrough isolating resistors 8| as shown. The first delay line D1 shownat 88 is represented in dotted outline indicating that this line mayhave zero delay and therefore that the delay line may be omitted, ifdesired. The amount of delay increases in succeeding delay lines D2, D3,etc. by .186 microsecond, the time interval between the beginning of onepulse and the beginning of the next pulse. Thus the code pulse generatedon digit electrode 4! is delayed six full pulse periods longer than theone generated on digit electrode 4|, that on digit electrode 46 isdelayed five pulse periods longer, etc. A common amplifier 90 is shownas serving for the transmission of the various permutation code pulses.This may also inelude shaping circuits to standardize the code pulseamplitudes if desired.

Some of the distributed code pulses are represented in the timingdiagram of Fig. 4. At the end of thecoding cycle immediately prior totime 0 the main beam pulse is assumed to have set up a coding pulse oneach of digit electrodes 4|. 4'3, 44, and 47. Corresponding pulses I, 2,4, 5 and l are therefore represented in the timing diagram as they wouldbe sequentially arranged in time by the distributo delay lines. Anothersequence generated. near the 'end of the first l 3-microsec and intervalis shown by the pulses i, 2, 3'. andfi.

- Fig. 2 shows a modified type of electrode construction in which eightindividual electron beams are used, one of these, 64 in the figure,being the pilot beam and. the other seven being beam; deflecting; plateaof; which-theaupuer; Qne-.is.

shown. at' 32 f is; .usedas the: previous flatness The-manner of.connecting; into, the. circuimwould.

be thersame as-. described'imconnectionrwith. Rig. 3; ,the w'arious:coding.- beam controlling-. grids-obes in paralleled; I

What is claimeddsz. v

1.. 'ilhe method. of. generating. pulses in-v code. combinations. by;electronic. action. comprising; pigoducing. two :parallelelectron; beamssubjecttd lilr-edeflection under,- signal control, initially, ap plyingthesignal.controhtoonly the first. of said, beams. to. deflect. thesame.to. a. desiredv position dependent. unon, the, signal, amplitude.while.- blanking. thesecond; beam, establishing; by feed-- bacleaction,efiectiueupon thafirstbeam, ,VOItage. conditions determinative. of thedeflection. oi, the second. beamwhen: exis-tent,, such. as to."causethe. second. beam to ,assuma-av position.limited. to. electrode.areas. determinative: of. one. particular code group.and.theneafter.and. while :said condi tions. are. stillmaintainedturning on saidsecondi beamifor a time corresponding to the le-ngth'ofcodev pulse to.be. generated.

2. In; an electrombeam. pulse code, producer, meanato,1r0duc.e.;t\voelectron beams parallel to each other, common, ,defiectimg; means, ionboth. beams. aquantizin grid. losatedr in,-.the pathof. the-first.orpilotbeam but out, oi the pathr-ofi the.

second. or. main: beam, code. generating; target.

areasin positionizo.beengagedby. said main beam. when deflected bydifferenuamounts-,, afeedback; control. from. said: quantizin g,. g-ridto J said. deflect. ing, means. for. establishing; and maintaining;proper, registry oil said, mainabeam with. thez particular. target.areas-producing. the. desired: code,- and means-tosuppresssaidmai',beanudurine; the time such pnopecregistry is.being, established:by;-said fecdbackhcontml beam during, the. timessuchspropert registrypis.b.e.- ing..maintained=.,

3. In, an. electronlbeam; coding: tuba, a main: beam, a.pilot-b.eam,-,.common-.deflecting means; for saidv beams,. a.quantizine. grid in; thevpath saidz pilot. beambutoutiet.the:-p.ath-.ofi said'main, beam; a feedback.beamacentering:controlfrom-said: grid-. to:saiddefiectingmeans;coderdetermining;target. areaspositioned to be engaged-mysaidmainrbeam; when deflected bysuccessiveramounts; said areas coordinated spatially with the gridapertures. suchthat said control centers,the-mainbeamonrespectivecode-determining areas, and means: to suppress said mainbeamzindependently of'said pilot beam.

4.. In: an electron beam codingtube, means: to-produce amain beam, aplurality of. targetex-- posureareas arranged in patterns accordingto-a" code, means to defiectthe main beam under con-- trol of asignalvoltage'to-becoded, so as toibe intercepted by certain of saidexposure areas. depending upon the signaii amplitude, meanszto suppresssaid beamexcept for times duringwhich; codes are to be generated;means-"to produce: a: pilotbeam, special target exposure: areasar buttoz turn. onsaid main.

rangedlopposite said'plurality of exposure: areas,

means:including, said. deflecting means, for moV-e ing said pilotbeams-to, one; of saidspecial areas underi-contr l =;ot,a-.signal toibecoded, while said means. also including, a. feedback control from.

ing proper; negister: of said main; beam-with;.said. certain exposureareas. 1

5. In; an electronbeam; coding tube-, a coding element... havingagpluralitm undefined areas! each representing.;a-,sig;nal amplitude,means, ionestabdefieetingsaid .bea-m: in accordance; with. theamplitude. of; a ignal to. becoded: tothe ,corresponding one ot= said;defined. areasmeans; for limiting thQzDOSSiblC: positions. ofisaid beam-to;.those. corre,

, spending; to.-said.;- defined areas saidmeanscom prisinameam; forgenerating, an auxiliary; electron, beam-within the-tube, a. quantiaingqelectrode dis posed-beside .saidcodingelement. to interceptisaidauxiliary; beam;.. andhaving openings positioned.oppositesaid-.,d;efined-. aneaa, said auxiliary; beam being deflected. bthe. signal, to. be. coded acting;

through .t'he' n eansefor deflecting. theifirstemen tioned; beam. and,.-meansi for feedin back from.

said. quantizing. electrode; a. controls voltage. for. applicationtozsaid deflecting-means;

6.. A-bcam, tubeefors producing permutation; code. pulses, comprisingmeans to project. a-main beam and a pilot. beam side by, sidawithin andalong said. tube, common deflecting means for bothbeams, aseparateand-independently controllable. beam. suppressing; meansforsaid mainbeam, said maintbeam. being many times. wider than its yer ticalorthicknessdimension, targetexposure areasarranged in horizon-tall rowsone above. another,

conformingto. a, permutation-code, a vertical column of othertargetexposureareas each horizonpliedwith deflection, controllingvoltages to 1 move bothlof. saidLbfiamsio..be...interc.epted. by. anygiven.

one. of said horizontal...rowssof. aiieasand. the: aligned therewithrespectively, and means: operating response, tointerception of. thepilot. beam said. latter; area, for. maintaining. said. beams.in..r,egi'ster,= with: the areasto. Whi'chthey.were;moyedrespeotively..,

'7. Iman. electronbeam coding, tube. means; to.

produce, a main beam, andl to project. the. beam.

toward a targetarea means. to) deflect the, beam.

to, difierentpositions. Qnsaidtarget. areadeuend ent, upom the... amplitd e of an. impressed; signal,. said target. area havinma difierentexposure; pat.- t'erni'or each different a mp litude, of. signalto be; codc'-,d',, endl'imeans. tnrediicez ambig ut incod'i'ng.compnisi'nzgmeans. to producegandproj ectla. pilot. beam;saidldeflectingsmeans. for. th main; beam alsoideflectingthe,pilotlbeamhy thes ameamount speciaLtarget. exposure areas arranged, to.be? in, tercent'edlby, saidqpilotibeam.vandiihdiiridually cor.-responding.,with' the difierentexposure area pat}; ternsmeansoperatihgin responseto.interception; of... saidlpilotbeamby a. given. one .101said special. exposure. areas tor feed-ins. backayoltage td saidldefiesting' means, such as-to. maintaimsaid. main,- beameisteswitn the.corresponding. exposure. area; nattern and an schemata, and.independently controllable; bean'risupnressing; means. fcn said.vmaimbeama- 8:; A3. two-beam. electron: coding tube: including;

mainbeam-is: suppressed. to predetermine thepo- I sition to. be assumed.bag-said;main-beam said last lishing; am eleetronpbeam in; said, tube,means for.

the;v individual areas being arranged. in a ,pattern.

main target areas and pilot target areas aligned therewith, means forinitially directively projecting a pilot beam, under control of a signalvoltage to be coded, against one of said pilot target areas whilemaintaining the main beam suppressed, a feedback control from said pilottarget for the pilot beam for properly centering the pilot beam on saidone pilot target area, and means subsequently operative under control ofsaid signal voltage and feedback centering control for projecting themain beam against the respectively aligned main target area.

9. A two-beam electron coding tube including code determining targetareas and guide target areas aligned therewith, means to produce a pilotbeam and a coding beam, beam deflecting means, means for initiallysuppressing said coding beam and for causing said deflecting means todirect said pilot beam, under control of a signal voltage to be coded,against one of said guide target areas, a feedback control from saidguide target area to said deflecting means for establishing properregister of said pilot beam on said guide target area, and meansincluding said deflecting means subsequentl operative under control ofsaid signal voltage and feedback control for projecting said coding beamagainst the corresponding codedetermining target area.

ROBERT L. CARBREY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,108,097 Bedford et a1 Feb. 15,1938 2,179,205 Toulon Nov. 7, 1939 2,180,944 Linsell Nov. 21, 19392,289,205 Nagy et a1 July '7, 1942 2,449,339 Sziklai Sept. 14, 19482,451,484 Gould et a1 Oct. 19, 1948

